Although several magneto-optic recording schemes have been proposed, the scheme of most interest with respect to the present invention depends on the existence of a recording medium displaying two effects. The first is the Kerr magneto-optic effect. A material which displays the Kerr magneto-optic effect produces a change in the polarization of light which reflects from a magnetized area of the material. For example, polarized light which falls on a magnetized area will undergo rotation of the polarization plane upon reflectance from an area magnetized in a given polarity A rotation in the opposite direction is produced by an oppositely-magnetized area.
The second effect is the effect under which media have a high coercive force at low temperature and a low coercive force at high temperature; i.e., under the second effect the media are more easily magnetized at high temperatures than at low temperatures.
In a magneto-optic system, data can be recorded on the medium by using a heat source to heat an area of the medium above the temperature at which the medium is magnetizable and exposing the heated medium to a low global magnetic field Upon cooling, the area which was heated and exposed to the magnetic field will be magnetized. Initially, the medium is normally bulk polarized in one direction. Thereafter, it is possible to detect whether a given area of the medium is or is not magnetized in a desired magnetic polarity by reflecting polarized light from the area and detecting rotation of the light. In this way, areas of a medium can be designated as marks for representing binary digits or bits and the binary value of a mark can be assigned according to the presence or absence of predetermined direction and/or amount of rotation of polarized light, which will correspond to an upward or downward magnetization of the area of the medium. By reversing the polarity of the bias field and heating a written area the area will return to the original bulk-magnetized direction. In this way, one can erase old information. Magneto-optic systems generally of this type are described in Mark H. Kryder "Magneto-Optic Recording Technology", J. Appl. Bhys. Vol. 57 No. 1, pp. 39113-3918 and Nobutake Imamura, "Research Applies Magnetic Thin Films and the Magneto-Optical Effect in Storage Devices", Journal of Electrical Engineering, March 1983, pp. 100-103.
A system of this type requires at least four parts: a medium displaying both of the above-described effects; a heat source for heating an area of the medium to the magnetization temperature; a magnetic field source; and apparatus for detecting whether a given area of the medium has been magnetized with a given polarity. Each of these four components can be provided in a number of forms and configurations.
Particularly preferred in connection with the present invention is a magnetic field source which includes a permanent magnet and a heat source which is a laser. A magneto-optical disk exerciser using a permanent magnet is briefly described in K. Ohta, et al. "Magneto-Optical Disk With Reflecting Layers", SPIE Vol. 382, pp. 252-259.
In a system in which it is desired that marks be not only writable to the medium but also erasable therefrom, the apparatus should be capable of producing a magnetic field in either of two polarities, e.g. substantially perpendicular to the plane of the recording medium and directed upward or substantially perpendicular to the plane of the recording medium and directed downward. In this way, it is possible to designate a convention by which one of the two magnetic polarities represents a mark and the other of the two magnetic polarities represents the absence of a mark. One method of providing a magnetic field in either of the polarities is to use an electromagnet. Although an electromagnet provides for easy control of magnetic polarity, it typically requires expenditure of high amounts of energy, particularly when it extends across the entire expanse of the recording medium (e.g. as is sometimes done to reduce weight of a movable carriage). Low energy consumption is particularly important in data storage devices used in connection with small computer installations such as personal computers.
An alternative to the use of electromagnets is the use of a permanent magnet device. Permanent magnet devices which have thus far been proposed, while they may be useful in reducing energy requirements, suffer from several disadvantages. One disadvantage relates to the fact that such devices do not provide for a variable field strength of the desired-orientation magnetic field. The magnetic field strength is defined by the nature of the magnet and, in previous devices, cannot be varied without also varying the orientation of the magnetic field. Thus, as a permanent magnet is moved or rotated from a position where it produces the first magnetic polarity, the field strength of the recording medium will change but also the orientation of the magnetic field at the recording medium will change. In previous permanent magnet devices, it has not been possible to produce a magnetic field at the recording medium which is both of a desired orientation, for example, always perpendicular to the recording medium, and of variable intensity. Having a magnetic field which is not of variable magnitude when in the desired orientation is disadvantageous because the magnetic characteristics of the recording medium may not be completely homogeneous across the entire recording medium surface, thus requiring either varying field intensities at various portions of the recording medium, or varying laser intensity.
A second shortcoming of previous permanent magnet devices relates to the control for moving or turning the magnet. The amount of time required to change from one magnetic polarity to another magnetic polarity, at least in part, affects the speed and efficiency with which data can be written and erased on the recording medium. To achieve flexibility and efficiency in recording and erasing data, it would be desirable to be able to achieve reversal of the magnetic field in a short time period, and in a time period which is independent of other movements or portions of the apparatus.
Yet another disadvantage of previous permanent magnet systems relates to the physical configuration or arrangement of such systems. When a system is arranged or configured such that apparatus (including permanent magnets, optical carriages or other apparatus) are disposed on both sides of a planar recording medium, the total height or space required to accommodate such an apparatus is greater than would otherwise be required. Such a configuration is provided in order to achieve the necessary field strength and orientation at the recording medium (while avoiding magnetic interference with other portions of the apparatus) by positioning the magnet close to the medium. Position of the magnet close to the medium provides reduced overall field strength requirements and/or provides for a lower-mass permanent magnet, while reducing energy requirements for turning the magnet. Reduction of height requirements is particularly important in recording systems used in conjunction with a computer system which is physically small. A typical personal computer system accommodates a disk-type recording apparatus in a space having a height of only about 3 inches (about 8 centimeters) or, in a "half-height" configuration, of about 11/2 inches (about 4 centimeters).
In view of the foregoing, it would be advantageous to provide an erasable magneto-optic data recording device which achieves relatively low energy consumption, produces a variable field-strength magnetic field in a desired orientation, is capable of magnetic polarity reversal in a time period which is short and which is independent of the movement or properties of other portions of the device or system, and/or is configured to minimize or reduce the height or volume requirements for the data recording apparatus or system.